Electroplating Chemical Leveler

ABSTRACT

Presented herein is a method of processing a device, comprising providing an electroplating bath having a leveler, the leveler having a total nitrogen-to-total carbon (TN/TOC) ratio of about 15% or less, bringing a substrate into contact with the electroplating bath, the substrate having a recess formed therein and electroplating the substrate to create a feature substantially free of voids in the substrate recess. Electroplating the substrate is performed for a time period about as long as an electrical response peak of the leveler, and optionally for at least 30 seconds. The leveler may optionally have at least one ingredient free of nitrogen and having a leveling functionality. One ingredient may be a benzene ring free of nitrogen. The leveler TN/TOC ratio is between about 3% and about 15%.

This application claims the benefit of U.S. Provisional Application Ser.No. 61/778,290, filed on Mar. 12, 2013, entitled “ElectroplatingChemical Leveler,” which application is hereby incorporated herein byreference.

BACKGROUND

As modern integrated circuits shrink in size, the associated featuresshrink in size as well. As transistor shrink, features such as throughvias and other electroplated elements shrink in size as well. In manyinstances, various layers of circuit on chips, dies, in packages, onPCBs and other substrates are interconnected between various layers byway of vias. Typically, the vias are connected to traces or otherconductive structures to route electrical signals through dielectriclayers. One way of forming a conductive via in a via opening is to formthe opening and then plate a conductive metal in the inside of theopening. In some instances, copper, gold, aluminum or other material areplated in the via openings.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIGS. 1-4 are cross-sectional views of a plating process for a viaopening;

FIG. 5 is a diagram illustrating electrical response time of a leveleraccording to an embodiment;

FIG. 6. is a diagram illustrating concentration dependent polarizationof a leveler according to an embodiment; and

FIG. 7 is a flow diagram illustrating a method of plating using aleveler according to an embodiment.

Corresponding numerals and symbols in the different figures generallyrefer to corresponding parts unless otherwise indicated. The figures aredrawn to clearly illustrate the relevant aspects of the variousembodiments and are not necessarily drawn to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the illustrative embodiments are discussed indetail below. It should be appreciated, however, that the presentdisclosure provides many applicable concepts that can be embodied in awide variety of specific contexts. The specific embodiments discussedare merely illustrative of specific ways to make and use the embodimentsof the disclosure, and do not limit the scope of the disclosure.

The present disclosure describes embodiments with regard to a specificcontext, namely controlling plating of recessed features. Theembodiments of the disclosure may also be applied, however, to a varietyof semiconductor devices, plating scenarios or other electrochemicaldeposition techniques. Hereinafter, various embodiments will beexplained in detail with reference to the accompanying drawings.

Plating of semiconductor device surface features is commonly achievedusing an electroplating technique, where an ionic metal in solution isdeposited on a substrate. In some instances, a metal may be depositeddirectly on a semiconductor substrate, or on a seed layer. An electriccurrent passed through the substrate and into the solution causesmigration of the metal in the solution to the surface of the substrate.Copper is a commonly used metal for electroplating due to its cost, wellunderstood plating properties, and adhesion to many differentsubstrates. Other metals used in the electroplating process may includegold, aluminum, tungsten, cobalt, nickel, chromium, silver, compounds oralloys of the same, or other conductive materials.

In order to control the rate of plating, surface properties and othervariables related to electroplating, various additives may be added tothe electroplating solution. Generally, the electroplating solution maybe a polar liquid, such as water, with a metal source, and, in somecases, an acid. A uniform deposition of plated material is required forthe development of complex conductor structures, especially for the highaspect ratio through hole plating. In electroplating smaller diameterholes and thicker panels, plating distribution becomes an importantconsideration. Material insufficiently deposited, for example, at thewalls of the through holes, or voids in pated structures result inrejection the device.

FIG. 1 illustrates an initial plating step according to an embodiment. Asubstrate 102 may have a recess 104 formed therein, and the recess 104may have sidewalls 112 and a bottom surface 114. The substrate 102 maybe exposed to an electroplating solution 116 having a leveler to form aplated layer 110. While the recess 104 of FIG. 1 is illustrated asextending partially through the substrate 102, the embodiments presentedherein may be applied to a recess or opening extending through thesubstrate to create a via opening or the like. Additionally, while thesidewalls 112 of the recess 104 are illustrated as being substantiallylinear and perpendicular to the top surface of the substrate, thesidewalls may be at an angle greater or less than 90 degrees to thesubstrate 102 surface, and may be curved, in linear sections, orotherwise non-linear.

Throwing power of an electroplating solution is a factor in thereliability of the electroplating process. Throwing power is the abilityof an electroplating solution to deposit a uniform thickness of platingacross a wafer or board when the electric field varies in differentregions of the electroplating bath. In general, plating processes thatprovide better leveling of the deposit across irregularities on thesubstrate surface and inside the through holes tend to worsen thethrowing power of the electroplating bath.

For example, in an embodiment, an electroplating solution for platingcopper may have copper sulfate (CuSO₄) and sulphuric acid (H₂SO₄). Thesulphuric acid disassociates the copper ions from the copper sulfate,allowing the copper sulfate to migrate to the substrate and form copperplate. However, it has been discovered that, with nanoscale features,the rate of copper plating may be so rapid that the surface finish ofthe copper is not acceptable. Additionally, when plating recesses in asubstrate, and in vias or high aspect ratio recesses in particular, therate of copper plating may be so rapid that the copper plate closes offover the top of a recess or opening before the interior of the recesshas completely filled with solidly plated material, resulting in a voidor cavity in the metal via. Such voids may be particularly undesirablebecause the lack of conductive material introduced by the void creates agreater electrical resistance, possibly bringing the feature out oftolerance.

In order to provide a smoother plated surface and reduce the errorsintroduced in small features by plating, many electroplating solutionsalso include additives such as brighteners, levelers, and suppressors.Organic compounds are added to an electroplating bath and act aslevelers and brighteners, increasing uniformity of metal deposition ondifferent regions of the PCB including through holes and recesses.Additionally, salts such as chlorides, may also be included in anelectroplating bath to as a brightener and to increase the deposition ofplating materials Organic compound such polyethylene glycol (PEG) or,alternatively, polyalkylene glycol (PAG) function as suppressors, whileorganic sulfides such as Bis(3-sulfopropyl)-disodium-sulfonate(C₆H₁₂Na₂O₆S₄) (SPS) work as accelerators.

A leveler frequently has ingredients with nitrogen functional group andis added to the bath at a relatively low concentration. Traditionalleveling involves the diffusion or migration of strongly currentsuppressing species to corners or edges of macroscopic objects whichotherwise plate more rapidly than desired due to electric field andsolution mass transfer effects. In the case of plating recesses oropening, the function of a leveler is to stop the excessive growth rateof copper over a feature to be filled while not otherwise perturbing theprocess.

However, large nitrogen concentrations found in levelers can cause voidsand cavities, particularly in high aspect ratio recesses such as viaopenings. In particular, nitrogen in levelers tends to interfere withadsorption of suppressors on the plated surface. The levelers displacethe suppressor, causing plating to occur too rapidly, and causing thecorners of via openings to plate faster than the plating can build upfrom the bottom, closing off the tops of the openings and creating voidsor cavities in the electroplated vias.

In an embodiment, the TN/TOC in the leveler may be between about 3% andabout 15%, and in another embodiment, may be about 5%. It has beendetermined that the reduced TN/TOC results in less interference with thesuppressor by the leveler and a longer electrical response peak in theelectroplating solution 116. In such an embodiment, the electricalresponse peak time of the electroplating solution 116 may be greaterthan thirty seconds, permitting recesses in the electroplating target tofill from the bottom 114 before plating at the corners 106 of a recess104 closes the recess 104.

A leveler may have one or more organic compounds acting as a leveler,and reducing the amount of a nitrogen containing compound may permitreduction of the overall TN/TOC ratio. For example, in some levelers, acompound having multiple nitrogen groups may be replaced with a compoundhaving fewer nitrogen groups, or with a compound being free of nitrogen.For example, benzotriazole (C₆H₅N₃) (BTA), Janus Green B, thiourea(SC(NH₂)₂, polyvinylpyrrolidone (C₆H₉NO)_(n) (PVP) and polyacrylamide(C₃H₅NO)_(n) components may be completely or partially replaced withnon-nitrogen hydrocarbons or hydrocarbons having a greatercarbon-to-nitrogen ratio, lowering the overall TN/TOC ratio. In anembodiment, benzene (C₆H₆) or a similar benzene ring or cyclichydrocarbon may be used as a constituent to reduce the overall levelerTN/TOC ratio. In such an embodiment, toluene, ethylbenzene, xylenecompounds, phenol or the like may be used to replace a nitrogencontaining leveler ingredient. The leveler may, in an embodiment, have aconstituent component that provides leveling capabilities orfunctionality and is free of nitrogen.

In an embodiment, the substrate 102 may be placed in an electroplatingsolution 116, with a leveler at, in an embodiment, a 5% TN/TOC ratio.Without a leveler and suppressor, the plating layer corners 108 tend toplate or grow most rapidly due to the relatively large surface area inrelation to plated layer 110 surface volume. However, with the levelerhaving a reduced TN/TOC between about 15% and about 3%, the suppressorfunction tends to dominate, slowing the plating process. A voltage maybe applied to the substrate and an anode in the electroplating solution116, and a plated layer 110 is formed over the exposed surfaces of thesubstrate 102. The recess 104 may be plated with copper or anotherconductive metal.

In one embodiment, the recess 104 may have a width of about 30 nm toabout 60 nm, and a depth of at least about 120 nm to about 200 nm, withan aspect ratio of about 4 to about 7. In an embodiment, electroplatinga recess 104 with an aspect ratio of 5 may fill the recess 104 to form avia or feature generally free of voids due to the bottom-up platingabilities of the reduced nitrogen leveler.

FIG. 2 illustrates an early state of plating in an embodiment. Using areduced nitrogen leveler permits the suppressor to prevent the platinglayer corners 108 from closing off the recess 104. In an embodiment, theplating layer 110 forms a bottom portion 110A without voids on thebottom surface 114 of the recess 104. The plating layer 110 also tendsto grow on the sidewalls of the recess 104 at a more uniform rate, theplating layer 110 on the sidewalls 112 being generally free of a platinglayer corner 108 that is substantially thicker than the plating layer onthe sidewalls 112. In an embodiment where the recess 104 is a throughvia opening, or otherwise extends through the substrate 102, thesubstantially uniform growth of the sidewall 112 plating layer 110 willfill the void from the sides, rather than the bottom surface 114 of arecess 104 since no bottom surface 114 exists. Uniform sidewall 112plating layer 110 growth on the sidewalls avoids voids or cavities in afilled via structures in such an embodiment.

FIG. 3 illustrates an intermediate stage of plating according to anembodiment. The plating layer bottom portion 110A continues to growwhile the plating layer corners 108 maintain a separation 302 permittingplating solution to escape the unplated portion of the recess 104.

FIG. 4 illustrates a plated recess according to an embodiment. A platinglayer 110 will form a substantially solid and substantially void-freevia 402 in the recess 104 by plating from the bottom or sides withoutclosing out the recess 104 by the plating layer corners 108 (FIG. 3). Inan embodiment, the plating layer 110 will have a substantially even orlevel top surface 404.

FIG. 5 illustrates the electrical response time of a leveler havingdifferent TN/TOC ratios. A leveler having about a 5% TN/TOC ratioexhibits an electrical response 502 that initially drops upon platinginitiation, and then peaks around the 35 second mark 508, resulting inan electrical response peak greater than 30 seconds. A leveler havingabout a 28% TN/TOC ratio exhibits an electrical response 504 with anelectrical response peak around the 10 second mark 506. In order toadequately plate the substrate 102, the substrate 102 may be plated fora period having a duration about as long as the electrical response peaktime of the leveler. For example, a substrate 102 may be plated in aleveler having about a 5% TN/TOC ratio for about 30 second or more,since the electrical response peak occurs around 30 seconds.

A reduced leveler nitrogen content further provides greater control overthe plating process and plating rate. FIG. 6 illustrates levelerpolarization as a function of leveler concentration. The polarizationcurves of a leveler with a 28% TN/TOC ratio at 2 seconds 604 and 4.5seconds 602 show roughly 300% greater deviation from an average 610 atdifferent leveler concentrations (shown in ml/L concentration) than thedeviation from the average 612 of polarization curves for a leveler witha 5% TN/TOC ratio at 2 seconds 606 and 4.5 seconds 608. Thus, a levelerwith a reduced nitrogen content, such as about 5% TN/TOC will tend tohave more predictable polarization levels at different levelerconcentrations.

FIG. 7 is a flow diagram illustrating a method 700 of plating using aleveler according to an embodiment. A substrate 102 may be provided inblock 702. The substrate 102 may have one or more recesses 104. Thesubstrate 102 may be applied, or submersed, in an electroplating bathhaving a leveler with reduced TN/TOC ratio. The reduced TN/TOC ratio ofthe leveler causes the electroplating solution 116 to plate at arelatively slow rate. The substrate 102 may be plated for at least about30 seconds in block 706.

Thus, according to an embodiment, a method of forming a device maycomprise bringing a substrate into contact with an electroplatingsolution having a leveler with a TN/TOC ratio of less than about 15% andelectroplating the substrate in the electroplating solution for a timeperiod about as long as an electrical response peak of the leveler,optionally for at least 30 seconds. The leveler TN/TOC ratio may bebetween about 3% and about 15%, and may optionally be about 5%. Theleveler may further optionally have at least one constituent ingredientfree of nitrogen and having a leveling functionality. The substrate mayhave a recess electroplating the substrate may comprise forming a via inthe recess, the via substantially free of voids. The substrate may beelectroplated with copper.

According to another embodiment, a method of processing a device maycomprise providing an electroplating bath having a leveler, the levelerhaving a TN/TOC ratio of about 15% or less, bringing a substrate intocontact with the electroplating bath, the substrate having a recessformed therein and electroplating the substrate to create a featuresubstantially free of voids in the substrate recess. Electroplating thesubstrate may be performed for a time period about as long as anelectrical response peak of the leveler, and optionally for at least 30seconds. The leveler may optionally have at least one ingredient free ofnitrogen and having a leveling functionality. One ingredient may be abenzene ring free of nitrogen. The leveler may have a TN/TOC betweenabout 3% and about 15%, and may optionally have a TN/TOC of about 5%.

A leveler according to an embodiment may comprise a hydrocarbon free ofnitrogen and having a leveling functionality, wherein the electroplatingleveler has an electrical response peak of at least about 30 seconds,and wherein the electroplating leveler has a total nitrogen-to-totalcarbon (TN/TOC) ratio of about 15% or less. The hydrocarbon of theleveler may be a benzene ring free of nitrogen, optionally benzene.

Although embodiments of the present disclosure and its advantages havebeen described in detail, it should be understood that various changes,substitutions and alterations can be made herein without departing fromthe spirit and scope of the disclosure as defined by the appendedclaims.

Moreover, the scope of the present application is not intended to belimited to the particular embodiments of the process, machine,manufacture, composition of matter, means, methods and steps describedin the specification. As one of ordinary skill in the art will readilyappreciate from the present disclosure, processes, machines,manufacture, compositions of matter, means, methods, or steps, presentlyexisting or later to be developed, that perform substantially the samefunction or achieve substantially the same result as the correspondingembodiments described herein may be utilized according to the presentdisclosure. Accordingly, the appended claims are intended to includewithin their scope such processes, machines, manufacture, compositionsof matter, means, methods, or steps.

What is claimed is:
 1. A method of forming a device, comprising:bringing a substrate into contact with an electroplating solution, theelectroplating solution having a leveler with a total nitrogen-to-totalcarbon (TN/TOC) ratio of less than about 15%; and electroplating thesubstrate in the electroplating solution for a time period having aduration about as long as an electrical response peak time of theleveler.
 2. The method of claim 1, wherein the electroplating thesubstrate comprises electroplating the substrate for at least about 30seconds.
 3. The method of claim 1, wherein the leveler TN/TOC ratio isbetween about 3% and about 15%.
 4. The method of claim 1, wherein theleveler TN/TOC ratio is about 5%.
 5. The method of claim 1, wherein thesubstrate has a recess and wherein the electroplating the substratecomprises electroplating the recess of the substrate and forming a viain the recess, the via substantially free of voids.
 6. The method ofclaim 1, wherein the electroplating the substrate compriseselectroplating the substrate with copper.
 7. The method of claim 1,wherein the bringing a substrate into contact with an electroplatingsolution comprises providing a leveler having at least one constituentingredient free of nitrogen, the at least one constituent ingredientfurther having a leveling functionality.
 8. A method of processing adevice, comprising: providing an electroplating bath having a leveler,the leveler having a total nitrogen-to-total carbon (TN/TOC) ratio ofabout 15% or less; bringing a substrate into contact with theelectroplating bath, the substrate having a recess formed therein; andelectroplating the substrate in the electroplating solution, theelectroplating the substrate comprising forming an electroplated featurein the recess, the feature substantially free of voids.
 9. The method ofclaim 8, wherein the electroplating the substrate is performed for atime period having a duration about as long as an electrical responsepeak time of the leveler.
 10. The method of claim 8, wherein theelectroplating the substrate is performed for at least about 30 seconds.11. The method of claim 8, wherein the leveler has at least oneingredient free of nitrogen, the at least one ingredient further havinga leveling functionality.
 12. The method of claim 8, wherein the levelerhas at least one ingredient that is a benzene ring free of nitrogen. 13.The method of claim 8, wherein the leveler comprises at least benzene.14. The method of claim 8, wherein the leveler TN/TOC ratio is betweenabout 3% and about 15%.
 15. The method of claim 8, wherein the levelerTN/TOC ratio is about 5%.
 16. An electroplating leveler, comprising: ahydrocarbon free of nitrogen and having a leveling functionality;wherein the electroplating leveler has an electrical response peak of atleast about 30 second, and wherein the electroplating leveler has atotal nitrogen-to-total carbon (TN/TOC) ratio of about 15% or less. 17.The electroplating leveler of claim 16, wherein the hydrocarbon is abenzene ring free of nitrogen.
 18. The electroplating leveler of claim16, wherein the hydrocarbon is benzene.
 19. The electroplating levelerof claim 16, wherein the leveler TN/TOC ratio is between about 3% andabout 15%.
 20. The electroplating leveler of claim 16, wherein theleveler TN/TOC ratio is about 5%.